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proposed the world is made up of empty space and tiny particles called atoms

atomic theory of matter

the idea that matter is made up of atoms

Antoine Lavoisier

made measurements of chemical reactions in sealed containers and observed that the mass of reactants before a reaction was equal to the mass of the products after the reaction

law of conservation of matter

states that when a chemical reaction occurs matter is neither created nor destroyed but only changed

Joseph Proust

studied compounds and observed that the elements that composed them were always in a certain proportion by mass (the law of definite proportions)

John Dalton

proposed the atomic theory of matter in 1803

scientific method

systematic approach to answer questions and solve problems


the first step to solving a problem


a testable prediction to explain observation


an explanation based on many observations and supported by the results of many investigations

scientific law

a fact of nature that is observed so often that it becomes accepted as truth (does not explain why something happens)

J.J. Thomson

discovered that atoms were not tiny solid balls that could not be broken up into parts but are composed of electrically charged particles


atoms of an element that are chemically alike but differ in mass

Hantaro Nagaoka

proposed a model of an atom in which electrons orbited a central, positively charged nucleus

atomic number

the number of protons in the nucleus of an atom of an element

mass number

the sum of the protons and neutrons in the nucleus of an atom

Ernest Rutherford

conducted an experiment that showed that the atom is nearly all empty space with a small dense nucleus

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